For the last fifteen years, transcranial magnetic stimulation combined with functional magnetic resonance imaging studies has firmly established its reputation as a leading non-invasive technique for medical/neurological studies, diagnostic tool and even as a therapeutic device for many medical/neurological and psychiatric conditions, especially in the treatment of certain types of depression.
In a typical prior art arrangement, shown on FIGS. 1 and 2, a TMS coil 1 is positioned next to a scalp 2 of a patient over an area of neurological interest, for example, a part of the brain cortex, so that the stimulation area of neurological interest is within an operational reach of the TMS coil 1. A pulsed magnetic field from the TMS coil 1 induces an electric current in the part of the brain cortex, locally depolarizing its neurons. This stimulates a response from the local neurons, and oftentimes even neurons that are far away from the stimulation area of neurological interest. The response from the neurons may result in subtle changes in a blood oxygenation level in the affected areas, from which brain cortex activity can be inferred. Since the blood oxygenation level can be visualized using fMRI techniques, the visualization effectively amounts to the visualization of brain cortex activity, though indirectly.
For the purpose of visualization, the head of the patient is subjected to a strong static magnetic field, which causes polarization of the nuclear spins in the body. Electromagnetic radiation with a frequency equal to the Larmor frequency of the nucleus of interest, usually hydrogen nuclei (i.e. protons), further referred to as a MR transmit pulse, excites the selected spin system to a higher energy level, and causes the protons to precess around the direction of the static magnetic field, thus, emitting a weak electromagnetic radiation. We will further refer to the weak response of protons as the MR signal.
The MR signal is picked up by an MR receiver coil 3, positioned further away from the TMS coil 1 and the scalp 2 of the patient. The MR receiver coil 3 will be further referred to simply as an MR coil. After appropriate, and often sophisticated, computer processing of the MR signal, an image of the affected area of the brain cortex is formed. More detailed description of the TMS and fMRI can be found, for example, in U.S. Pat. No. 5,732,702 to Edgar Mueller entitled “Method and apparatus for functional imaging” issued on Mar. 31, 1998.
Although the above arrangement for combined TMS/fMRI studies has served the researchers reasonably well in the past, its shortcomings are quickly becoming a limiting factor in meeting new research and application demands.
A problem that requires particular attention of engineers is the low signal-to-noise ratio (SNR), which, in part, is caused by the weak MR signal, but also due to the fact that MR coils in previous combined TMS/fMRI studies had to be large enough to accommodate both, the head and the TMS coil. This required large sized MR coils having a large distance to the head, thereby reducing the achievable SNR.
Since increasing the MR signal itself does not seem to be feasible, because of physical limitations of the MR process, prior art offers various solutions, such as accumulation and subsequent statistical processing of the MR signal using sophisticated algorithms.
Unfortunately, the solutions offered by the prior art have been only partially successful, and have often introduced new problems, which are sometimes difficult to solve. For instance, the accumulation of the weak MR signal requires long measurement time, which may cause discomfort to a patient or be outright impossible for visualization of relatively fast dynamic processes, such as heartbeat etc.
Therefore, there is a need in the industry for further development of an improved TMS/fMRI system and method, which would provide further improvements to the TMS/fMRI studies, and avoid or mitigate the above noted problems of the prior art.